Poly(butylene terephthalate), or PBT, which is represented by the following formula: ##STR1## is an important polyester fiber; it is also an important crystalline thermoplastic resin. Polyester fibers, which show good resistance to most mineral acids and display excellent resistance to cleaning solvents and surfactants, are now the world's leading synthetic fibers. Crystalline thermoplastic poly(butylene terephthalate), along with other crystalline thermoplastic poly(alkylene terephthalates), such as poly(ethylene terephthalates), exhibits many favorable physical, mechanical, and electrical properties, and is considered one of the most important engineering plastics.
Poly(butylene terephthalate) is manufactured commercially by the so-called DMT (dimethyl terephthalate) or TPA (terephthalic acid) process, both of which typically use tetrabutyl titanate or tetraisopropyl titanate as the catalyst.
In the conventional DMT process for making poly(butylene terephthalate), dimethyl terephthalate (DMT) and butylene glycol (i.e., 1,4-butanediol, BDO) are used as the raw materials which are reacted at 220.degree..about.260.degree. C., using tetrabutyl titanate or tetraisopropyl titanate as the catalyst to effectuate a transesterification reaction and form an intermediate product, which is bis(2-hydroxybutyl terephthalate) (BHBT). The intermediate product bis(2-hydroxybutyl terephthalate) is then subject to a high temperature (250.degree..about.290.degree. C.) and high vacuum (less than 1 tort) to effectuate a condensation polymerization. After the reaction, the unreacted butylene glycol is removed to obtain the poly(butylene terephthalate) final product.
In the conventional TPA process, terephthalic acid (TPA) and butylene glycol are used as the raw material which are reacted at 220.degree..about.265.degree. C. to effectuate an esterification reaction, using tetrabutyl titanate or tetraisopropyl titanate as the catalyst. An intermediate product of bis(2-hydroxybutyl terephthalate) is formed after dehydration of the terephthalic acid (TPA) and butylene glycol as a result of the esterification reaction therebetween. The intermediate product of bis(2-hydroxybutyl terephthalate) is similarly subject to a high temperature (250.degree..about.290.degree. C.) and high vacuum (less than 1 torr) condensation polymerization. After the completion of the reaction, the unreacted butylene glycol is removed from the reaction product to obtain the poly(butylene terephthalate) final product.
In Netherland Pat. App. No. NL 86/2,460 (also appeared as EP 264,143; EP 87-201,541; and DE 3,765,514), it is disclosed an aromatic polyester blend, in which disodium diphosphate Na.sub.2 H.sub.2 PO.sub.4 is added to a blend of poly(butylene terephthalate) and bisphenol A polycarbonate as a stabilizer. In Japan Laid-Open Patent Application JP 61-31,455 (also appeared in JP 92-061,903), it is disclosed a process for melt-molding polymer composition by which metal phosphates and/or phosphites are added, as viscosity stabilizers, to blends of aromatic polyesters, including poly(ethylene terephthalate) and poly(1,4-butylene terephthalate), and bisphenol A or bromobisphenol A polycarbonates.
In U.S. Pat. No. 4,532,290, it is disclosed a stabilized polycarbonate-polyester composition, in which a thermoplastic composition containing a polycarbonate and a terephthalate polyester is melt stabilized with NaH.sub.2 PO.sub.4 and/or KH.sub.2 PO.sub.4. In Japan Patent 75-67,355, it is disclosed a polyester molding composition with good color stability containing poly(tetramethylene terephthalate), bisphenol A polycarbonate, butadiene-methyl methacrylatestyrene graff copolymer, and NaH.sub.2 PO.sub.4. The composition has exhibited good moldability, chemical resistance, mechanical properties, heat-distortion resistance, and color. None of these references teaches the use of phosphate or phosphite as a co-catalyst for use with tetrabutyl titanate or tetraisopropyl titanate, for the production of poly(butylene terephthalate) from dimethyl terephthalate.
In U.S. Pat. No. 4,780,527, Tong, Chen, Yuo, and Chang teach a catalyzed process for the esterification of terephthalic acid with 1,4-butanediol. The catalyst composition, which consists of organic Sn or Ti compounds and organic and/or inorganic salts, was found to provide higher reaction conversion and reduce the production the THF by-product. In Japan Laid-Open Patent Application JP 60-161,455, it is disclosed a process for making polybutylene terephthalate with high impact resistance by reacting terephthalic acid and 1,4-butanediol with alkali metal hypophosphite and vinyl copolymer robber. In an article which appeared in J. Appl. Polym. Sci., 45(20), pp. 371-3 (1992), it is disclosed that the formation of THF in the preparation of poly(butylene terephthalate) from butanediol and terephthalic acid can be reduced by the addition of potassium and sodium salts. Again, none of these references teaches the use of alkali metal phosphate or alkali metal phosphite as a co-catalyst for use with tetrabutyl titanate or tetraisopropyl titanate, for the production of poly(butylene terephthalate) from dimethyl terephthalate.
In a co-pending application Ser. No. 08/413,025, which was by a co-inventor of the present invention and has been assigned to the same assignee of the present invention, it was disclosed an improved tetrabutyl titanate based catalyst composition which contains: (1) between about 0.01 PHR and about 1 PHR of a primary catalyst tetrabutyl titanate, and (2) between about 0.001 PHR and 1 PHR of an alkali metal phosphate or an alkali metal phosphite co-catalyst. In the above designation, PHR stands for parts per hundred parts, by weight, of dimethyl terephthalate. In that application, the alkali metal phosphate can be a phosphate salt containing one, two, or three metal groups, and the alkali metal phosphite can be a phosphite salt containing one or two metal groups. Examples of the alkali metal phosphates include tri sodium phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate. Examples of the alkali metal phosphites include disodium hydrogen phosphite, and sodium dihydrogen phosphite. It has been demonstrated in this co-pending application that the polymerization rate can be increased by 10 percent using the cocatalyst disclosed therein.
Because of the high commercial value and large volume of poly(butylene terephthalate), any incremental improvement in the production rate thereof is very significant. Therefore, it is important to improve the performance of the tetrabutyl titanate (or tetraisopropyl titanate) based catalyst composition. Furthermore, an increased production rate would also reduce the time during which the expensive tetrabutyl titanate catalyst is subject to the possibility of being poisoned, per unit volume of poly(butylene terephthalate) produced. This can also result in important cost savings in the production of poly(butylene terephthalate).